Heretofore, for glass substrates for use in applications that require light transmission therethrough, such as glass for building materials, glass for automobiles, glass for displays, optical elements, glass substrates for solar cells, glass for shop windows, optical glass, lenses for glasses, etc., there is a case of forming an anti-reflection film on the surface of the glass substrate for increasing the light transmittance. For example, for obtaining high-permeability glass members, an anti-reflection film has heretofore been formed with a fluoride film such as an MgF2 or a hollow SiO2 film on the surface of the member according to a method of dry coating such as vapor deposition, sputtering or the like, or a method of wet coating such as spreading, spin coating or the like.
However, the functional film differing in property from the glass substrate is formed, which is, therefore, problematic in that the adhesiveness between the glass substrate and the functional film is poor and the film may readily peel by the operation such as wiping. Accordingly, a method of bringing a fluorinating agent into contact with the surface of a glass substrate to thereby form a porous structure on the surface of glass (hereinafter this may be referred to as “etching”), thereby forming an anti-reflection film thereon, is known (Patent Documents 1 to 3).
This may be presumed to be as follows: on the glass surface, the fluorine compound could react with SiO2 which forms the network structure of glass, to thereby form SiF4 (gas), and as a result, the remaining part that has lost the network could be a silicofluoride to form the porous region on the surface.
The above-mentioned Patent Document 1 mentions, as the fluorinating agent, a fluorine elemental substance (F2) or a fluorine compound capable of forming a bond between a fluorine atom and a metal atom by cutting the bond between the oxygen atom and the metal atom in the glass network, for example, hydrogen fluoride (HF), silicon tetrafluoride, phosphorus pentafluoride, phosphorus trifluoride, boron trifluoride, nitrogen trifluoride, chlorine trifluoride, and mentions that, among these, the fluorine elemental substance is most preferred as highly reactive as it is and capable of shortening the reaction time. Regarding the concentration of the fluorinating agent, the document describes that, in a case where the concentration is too low, then the reaction speed is low and the treatment time is long, but in a case where the concentration is too high, then the reaction speed is high and the reaction control is difficult, and further describes that, by raising the temperature of the gaseous fluorinating agent and/or by increasing the pressure thereof, the fluorine atom concentration on the glass surface can be increased; and concretely, for forming the porous structure, a fluorine elemental substance is used as the fluorinating agent, and in a case where the F2 concentration is 20 mol %, then the surface treatment is carried out at from 20 to 80° C. for from 1 to 8 hours, and in a case where the F2 concentration is 2 mol %, then the surface treatment is carried out at from 550 to 600° C. for 15 minutes.
Patent Document 2 describes that, by controlling the hydrogen fluoride concentration on the glass surface to be 1 mol % or less, the surface of glass can be subjected to fluorination treatment with excellent adhesiveness at low cost, without worsening the surface properties owing to any excessive etching effect, and also describes that, for controlling the hydrogen fluoride concentration to be 1 mol % or less, hydrogen fluoride is not used as the fluorinating agent. In Patent Document 3, a glass substrate is surface-treated at from 10° C. to 60° C., using a gas that contains hydrogen fluoride and water.